The present invention relates to compositions comprising polymers whose solubility characteristics can be changed by incubation and particularly poly (N-(2-hydroxypropyl)methacrylamide mono/di-lactate) interpolymers. Another aspect of this invention is the application of such temperature sensitive polymers as release systems of biologically active compounds. The polymers of the present invention, comprise monomers which have modifiable functionality. The functionality of the monomers can for example be modified by the presence of hydrolysable groups. The modification is effected by the incubation, leading to a change of the water solubility characteristics of the polymer. The polymers used in the present invention contain hydrolysable chemical groups. As a result the polymer's solution characteristics, specifically its lower critical solution temperature (LCST), change upon incubation.